(a.) Field of the Invention
This invention relates to insoluble lead dioxide coated graphite anodes for electrowinning materials.
(b.) Description of the Prior Art
Lead dioxide is suitable as a coating on anodes due to its relatively show rate of erosion in many electrolyte systems. In contrast, the base materials on which the lead dioxide is commonly deposited are easily eroded by many electrolyte systems. Access to such base materials having lead dioxide coatings commonly occurs via pinholes or other defects resulting from the coating process. In addition lead dioxide has shown a tendency to flake or crack during normal handling due to its inherent brittleness and its poor adhesion to the base anode material.
A number of methods have been proposed to overcome the problems attending use of lead dioxide coated anodes. For example, U.S. Pat. No. 2,872,405 describes an improved anode comprising a metal screen on which there is electrodeposited a lead dioxide coating and the interstices of which are completely filled with lead dioxide. The anode has enhanced mechanical strength, less tendency to crack during handling, and is less susceptible to breakdown during operation.
U.S. Pat. No. 2,945,791 proposes to improve the quality of the lead dioxide coating on graphite or carbon electrodes by electrodepositing the coating from a special lead nitrate electrolyte employing a specific sequence of operating steps, including a water soak of the substrate material to eliminate pinholes in the coating and agitation during electrodeposition to wipe bubbles off the base material. U.S. Pat. No. 3,463,707 employs an electrolyte in which high acid levels are maintained to electrodeposit a thin and then a second thick layer of lead dioxide on the anode in order to produce a better product. In U.S. Pat. No. 3,880,728 lead dioxide is electrodeposited on a titanium substrate following deposition of an intermediate carbide, boride or silicide layer. U.S. Pat. No. 4,026,786 describes electrodeposition of lead dioxide onto titantium substrates from electrolytes containing high levels of nitric acid in order to produce more satisfactory anodes without necessity for precoating or use of fluoride additives. Finally, U.S. Pat. No. 4,159,231 employs alternating current in conjunction with direct current during lead dioxide deposition to extend anode life.